The present invention relates to a scanning tunneling microscope and, more particularly, to a scanning tunneling microscope (to be referred to as an STM hereinafter) which detects luminescence caused by electrons injected into, e.g., a material or a very small region in a structure embedded in a material, and measures the spectrum of the light or a spatial distribution image of luminescence corresponding to a detection position, thereby allowing observation of the characteristics of the small material or the very small region of the small structure with a high precision and a high resolution.
With the recent trend toward higher degrees of integration and higher speed of ICs, a rapid reduction in element size has been witnessed. Recently, great demands have arisen for realization of a so-called quantum-effect device, e.g., a high-speed, high-performance electronic device or optical device, as a circuit element of the next generation. Such a device is based on a quantum-mechanical effect obtained by confining particles such as electrons, holes, or excitons in a semiconductor structure (to be referred to as a quantum structure hereinafter) having a size on the nanometer order, which is almost equal to the size of such particles.
On the other hand, with the advances in the semiconductor crystal growth/process techniques, a structure having a size almost equal to that of the above particle having a size on the nanometer order can be formed. In a structure having such a small size, a particle exhibits a remarkable quantum effect. It is, therefore, expected that a quantum-effect device be realized by controlling/using such characteristics. Since the characteristics of a quantum-effect device are greatly influenced by the dimensional precision and characteristics of a quantum structure, in order to realize a good quantum-effect device, evaluation of a quantum structure is important.
Excitons confined in a quantum structure emit light having a spectrum sensitively reflecting a confined state. Therefore, measurement of this emission is an effective means for obtaining detailed information about the characteristics of the quantum structure.
In general, a quantum structure is formed by being surrounded by materials having different physical properties. For this reason, a quantum structure is often embedded in a sample.
As conventional methods and means of optically measuring a quantum structure by using luminescence from excitons, a photoluminescence (PL) method using light as a means for generating electron-hole pairs, a cathode luminescence (CL) method using high-energy electrons as a means for generating electron-hole pairs, a photon scanning microscope, and the like are available. However, these methods are insufficient for measurement of a very small region of a quantum structure for the following reasons.
In both the PL and CL methods, problems are posed in terms of exciton diffusion length and probe diameter (excitation beam diameter), resulting in a deterioration in spatial resolution.
More specifically, in both the PL and CL methods, excitons are diffused to places far away from an exciton generation place within a time interval between the instant at which they are generated and the instant at which they emit light. The range of diffusion is equivalent to a spread of a submicron or more. This makes it difficult to improve the spatial resolution. In addition, since excitons tend to be diffused to a region having a lower energy (thicker quantum structure) than a place where they are generated, the intensity of luminescence from a thick quantum structure is inevitably increased. That is, an emission distribution does not always reflect a thick distribution accurately.
The influences of an excitation beam diameter will be described next.
In the PL method, visible light or infrared light having a longer wavelength than visible light is generally used as excitation light. The wavelength of such light is about 0.5 to 1 .mu.m. For this reason, the spot size of this excitation light cannot be reduced to a value smaller than the wavelength of the light. In addition, in consideration of a spread caused by factors based on the arrangements of other units, it is difficult to set the beam diameter on the micron order in practice.
In the CL method, since the electron energy is high, electrons are scattered in a sample over a wide range, and the scattered electrons also generate electron-hole pairs. For this reason, even if the electron beam diameter is reduced, it is difficult to reduce a region (generation volume) in which electron-hole pairs are generated. In addition, since a high energy and a large current are concentrated on a measurement region, the measurement may deteriorate or change in state.
As described above, in these methods, since the exciton diffusion length or the excitation beam diameter is large, it is difficult to improve the spatial resolution beyond the submicron order. Therefore, the methods cannot be applied to measurement of a very small region on the nanometer order.
Under these circumstances, an STM has been developed as a means for injecting an excitation beam into a very small region and measuring luminescence from this region.
A photon scanning microscope as another means for optically measuring a very small region is designed to detect light (evanescent light) locally existing at a place very near a sample surface with a transparent probe, which light is produced by excitation light emitted from a laser source different from the probe (R. C. Reddick, R. J. Warmack, and T. L. Ferrell, "New Form of Scanning Optical Microscopy", Phys. Rev. B39, pp. 767-770 (1989)). Since this instrument does not use injection of electrons into a sample, the probe is made of a glass material selected in consideration of only transparency while conductivity is neglected. In general, a light transparent material which can be used for a structure like a probe has good insulating properties but has no mechanism for supplying a current to a probe tip. In addition, as a modification of the photon scanning microscope, a photon scanning microscope using a tunnel current to control the distance between a sample and the probe is also available. In this instrument, a metal is deposited on the surface of the probe having good transparency to a thickness enough for shielding light, and a very small opening is formed in the tip of the probe to detect light entering through the opening. The instrument uses a tunnel current only for measurement/control of the distance between the probe and a sample surface, and the tunnel current does not contribute to luminescence. In addition, since the spatial resolution of measurement is determined by the size of the opening of the probe tip, the opening cannot be expanded, resulting in a very low light collection efficiency. In the photon scanning microscope, a region where measurement can be performed is theoretically limited to a place very near a sample surface. It is, therefore, impossible to evaluate a quantum structure.
This STM uses the property that a tunnel current injected from the probe of the STM into a sample concentrates on a very small region in the sample. In the STM, electrons are injected from the probe tip into a sample surface upon tunneling through a small spatial distance. Since the energy of tunneling electrons is much lower than that of an electron beam used by an electron microscope, electron-hole pairs can be generated to cause luminescence in a very small region.
In a conventional STM, however, since the probe is used to only supply tunneling electrons, the probe is just a metal lump and does not have a function of collecting light. A reflecting mirror arranged separately from the probe serves as a means for collecting emitted light. Since light radiated from a sample surface almost in accordance with the cosine law, light radiated in a direction perpendicular to the sample, i.e., the direction of the probe, has the highest intensity. According to the conventional structure, since this light portion having the highest intensity is shielded by the probe and cannot be used, the ratio of light which can be collected is low. Consequently, detected light is weak, and the signal-to-noise ratio is low. It is, therefore, difficult to perform high-precision measurement.
As described above, the photon scanning microscope cannot be theoretically used for optical evaluation of a quantum structure, whereas a required spatial resolution cannot be obtained in performing optical evaluation of a quantum structure by using the PL and CL methods. The STM is capable of measuring a quantum structure. However, with the conventional arrangement, a photodetection signal is weak, and high-precision measurement is difficult to perform. Therefore, it is difficult to measure the optical characteristics of a very small region of a quantum structure with a high precision and a high sensitivity by using any of the conventional instruments.